Line current reversal is one technique employed in telephone ringing signal generators for converting a transformer coupled alternating current signal into a telephone ringing signal. The transformer coupled alternating current signal is usually a 60 Hz line signal and the ringing signal is a 20 Hz signal, although 30 Hz ringing signals are also in use.
Ringing signal generators of this type incorporate coupling transformers with large iron cores and complex magnetic structures. This is necessitated by the requirement that the transformer accommodate low frequency circulating currents in its magnetic path. The low frequency circulating currents are present at the ringing signal frequency (20 Hz or 30 Hz) and at harmonics of the ringing signal frequency. Presence of these low frequency circulating currents is caused by the lack of suitable isolation between the coupling transformer and the ringing signal generator to inhibit feedback of the ringing signal through the coupling transformer.